The present invention relates to an HVAC system having displacement with induction that meets or exceeds accepted standards for acoustical levels, especially for educational facilities. Designers of such a system face considerable challenges in applying equipment that complies with the acoustical levels specified in ANSI/ASA Standard S12.60, adopted in 2002. Like all ANSI Standards, the imposition of S12.60 is voluntary, so its evolution as a regulatory document is likely to be a gradual process. Notwithstanding such evolution, the instant invention is prepared to meet the challenges.
ANSI/ASA Standard S12-60-2002 developed as a result of engineers seeking to reduce noise levels in the nation's classrooms. Its objective was to ensure that the educator's speech could be clearly understood by almost all of the students within the classroom, when delivered at levels that do not require electronic amplification nor cause undue vocal stress on the educator. The introduction of the Standard in 2002 was made amid a great deal of industry controversy as it stipulated the maintenance of space noise levels not exceeding 35 dBA (about NC27) in core learning areas. Conformance to the Standard essentially precludes the use of unit ventilators and other packaged equipment within the classroom as sufficient insulation and/or isolation of these noise sources cannot feasibly be accomplished. Fan coils and heat pumps serving the classroom would also have to be located outside the space and ducted to allow for an appropriate level of attenuation prior to discharge within the space. It should be noted that the 2003 ASHRAE Handbook (Applications) recommends that classrooms be designed for acoustical levels conforming to the Standard.
Almost all North American schools are served by mixed air diffusion systems. Mixed systems, though operating in a cooling mode, introduce conditioned air at discharge velocities of 250 to 300 fpm, (1.25 to 1.5 m's) and supply air temperatures as low as 55 degree F. They rely on relatively high discharge velocities to entrain room air and mix it thoroughly with the supply air near the point of discharge. Residual air movement caused by this induction creates well mixed conditions and uniform temperature and contamination levels throughout the space. This contaminant removal method is referred to as dilution ventilation. It should be noted that the ideal upper limit of ventilation effectiveness in a mixed system is 1.0. Space temperature control may be accomplished by                a) varying the delivered air volume at a constant supply temperature,        b) varying the supply air temperature at a constant air volume, or        c) varying both the supply air volume and temperature.        
Varying the supply air volume makes it almost impossible to maintain mandated outside airflow rates. To keep the air volume constant often results in increased energy usage at these constant air volume deliveries (typically 2.5 to 3 times the space minimum ventilation rate), and must be maintained at all times, regardless of load. Variations in space airflow rates also result in proportional variations in space RH during humid operational periods.
Displacement air conditioning has been considered in Europe as a method of providing high levels of comfort and ventilation effectiveness. It relies on natural stratification to transport conditioned air through the space. Cool air at 63 to 68 degree F. is supplied at very low discharge velocities (50 to 70 fpm) from low sidewall or floor based outlets. The low velocity is not sufficient to create significant entrainment of room air, thus the supply air maintains most of its thermal integrity as it falls and spreads across the floor. This air is confined to the lower extremities of the space by the warmer ambient air above it.
Occupants and electrical equipment in the classroom transfer heat to the ambient air by natural convection. This convective transfer (in the absence of random velocity vectors) results in the formation of thermal plumes along the boundaries of the heat sources which rise through the upper parts of the space (gradually increasing in volume as they rise) until they either encounter equally warm air or reach the overhead return outlet. Cool air from the floor, drawn upward as the plume forms, passes over the boundaries of the heat source, conditioning it and, in the case of the space occupants, providing the source of inhaled respiration. Exhaled air is warmer than ambient and is thus conveyed with the rising thermal plume directly to the upper portion of the space where it can be easily removed. No horizontal transport of the respiratory contaminants occurs in such a system. It is estimated that some 60% of the transmission of contagious disease in elementary schools is due to such airborne transmission.
Since an inductive air distributing system is a relatively new development, there is little prior art to help describe and understand how the system may be effective as an air handling system. One such U.S. patent is U.S. Pat. No. 6,569,010, to Miller et al., which teaches an air handling system that receives air from a primary air source and distributes that air in a room defining an enclosed space. The system is mounted in the ceiling of the room and generally comprises first, second, third, and fourth inductor units interconnected downstream to the primary air supply and which define first, second, third, and fourth areas, respectively, wherein the induced air flows through each of the inductor units, through a series of converging nozzles, and into each units, first, second, third, and fourth areas, respectively, and an air diffusing mechanism positioned adjacent to the first, second, third, and fourth areas which directs the induced air in first, second, third, and fourth directions, respectively. The inductors are generally arranged in a square configuration with the diffuser extending in the square space bound thereby.
Another prior system is described in U.S. Pat. No. 6,290,595, to Daunay, where the patent relates to a device for regulating the temperature of premises by a secondary air flow blown therein, and an upstream duct communicating with elements for supplying pressurized air, ending in a converging element emerging into a downstream duct communicating with the premises. The device is characterized in that a substantially rotating solid, capable of being positioned along the convergent element longitudinal axis, is arranged at least partially upstream of the outlet thereof, so as to define between the substantially rotating solid outer surface and the convergent element inner wall with a ring-shaped channel the size of which depends on the position of the solid relative to the wall.
Though the prior art is limited, it is acknowledged that displacement ventilation and/or conditioning for central type ventilation systems is known and used. Further, room air induction has been practiced in ventilating systems, particularly in Europe, but no system is known in the prior art that effectively combines these approaches in a low noise level, HVAC system in the manner of the present invention. The effectiveness of this combination will become more apparent in the following specification, especially when read in conjunction with the accompanying drawings.